Modern mobile communication systems use multiple channels, closely spaced over an assigned frequency band. In order to avoid intermodulation products and spectral regrowth, both in and out of band, it is essential that radio frequency (RF) power amplifier circuits used in these systems be highly linear. A high level of linearity is also required in single-channel transmitters which transmit a wideband, variable-envelope signal, such as a CDMA signal.
A major source of non-linearity is distortion, which occurs due to non-linear amplitude and phase response of the amplifier, particularly as power nears the saturation level. Third-order distortion non-linearities typically give the strongest intermodulation products, but higher-order products can be significant. One method of correcting for amplifier distortion, and thus improving linearity, is predistortion, in which a controlled, non-linear distortion is applied to the amplifier input signals. Predistortion circuitry is designed to give non-linear amplitude and phase characteristics complementary to the distortion generated by the amplifier itself, so that ideally, the distortion is canceled in the amplifier output over the entire signal bandwidth. A feedback connection is generally provided from the amplifier output to the predistortion circuitry, for use in adjusting predistortion coefficients for optimal linearization.
Various schemes have been proposed for digital-domain predistortion of RF power amplifier input signals. For example, U.S. Pat. No. 6,141,390, whose disclosure is incorporated herein by reference, describes a system that uses a straight inverse modeling scheme with orthogonal predictor variables to determine the inverse of the distortion caused by a power amplifier of a RF transmitter. The predistorter system determines complex predistorter coefficients based on the inverse modeling scheme, and stores the coefficients in a look-up table (LUT). The coefficients from the LUT are then used as the tap weights of a non-linear digital filter implementing the predistorter.
Another digital predistortion system is described in U.S. Pat. Nos. 6,549,067 and 6,580,320, whose disclosures are also incorporated herein by reference. A predistortion circuit samples the input to a non-linear amplifier, and multiplies the input by itself using mixers in order to generate various orders of distortion. Filters/time delay means are incorporated into the paths that generate the orders of distortion in order to produce phase and/or amplitude variation with frequency. The distortion orders are summed to provide the predistortion. The filter/time delay means can be implemented by adaptive filters in digital signal processing (DSP) circuits, which sample the output of the amplifier being linearized in order to obtain feedback for adapting the filter(s). The in-phase and quadrature parts of the input are separately digitally predistorted.